Chalk used in school classrooms comes in slender sticks approximately 9mm in diameter and 80 mm long. Lessons are often presented to entire classes on chalk-boards (or blackboards, as they were originally called) using sticks of chalk because this method has proven cheap and easy.
As found in nature, chalk has been used for drawing since prehistoric times, when, according to archaeologists, it helped to create some of the earliest cave drawings. Later, artists of different countries and styles used chalk mainly for sketches, and some such drawings, protected with shellac or a similar substance, have survived. Chalk was first formed into sticks for the convenience of artists. The method was to grind natural chalk to a fine powder, then add water, clay as a binder, and various dry colors. The resultant putty was then rolled into cylinders and dried. Although impurities produce natural chalk in many colors,
Chalk did not become standard in schoolrooms until the nineteenth century, when class sizes began to increase and teachers needed a convenient way of conveying information to many students at one time. Not only did instructors use large blackboards, but students also worked with individual chalkboards, complete with chalk sticks and a sponge or cloth to use as an eraser. These small chalkboards were used for practice, especially among the younger students. Pens dipped in ink wells were the preferred tool for writing final copy, but these were reserved for older students who could be trusted not to make a mess: paper—made solely from rags at this time—was expensive.
An important change in the nature of classroom chalk paralleled a change in chalk-boards. Blackboards used to be black, because they were made from true slate. While some experts advocated a change to yellow chalkboards and dark blue or purple chalk to simulate writing on paper, when manufacturers began to fashion chalkboards from synthetic materials during the twentieth century, they chose the color green, arguing that it was easier on the eyes. Yellow became the preferred color for chalk.
There are two ways of making chalk;
Molded chalk that is based on using calcium sulphate – plaster chalk , and it is soft, which as Jumbo chalk , round soft chalk
Extruded chalk , That is based on calcium carbonate chalk , This kind of chalk is dustless chalk. It is most commonly used on chalkboards in school, as it tends to be dense and "dustless".
Molding Process The moulded method , produces chalk which is softer than the extruded chalk, and is NOT dustless. Dry pigment and water are first mixed in preparation for the addition of plaster of Paris. Because plaster begins to set quickly, this process must be carefully timed and supervised. Before the mixture gets too thick, it is poured into a molding machine with many holes that are the exact size and shape of the finished pieces of chalk. Excess chalk "batter" is scraped off the tops of the moulds, and after a setting of five to eight minutes, depending upon the particular color being manufactured, the sticks of chalk are popped out onto a tray which is then stacked in a large drying rack.
Extrusion Process To produce extruded chalk, the makers first stir together, by machine, several white powders. The most important of these is calcium carbonate and water-washed clay. If the chalk is to be colored, the particular color is likewise added in the form of dry, finely ground pigments. After all of the powders are well mixed, they are transferred to another large machine where a liquid "binder" is poured in to hold the dry particles together.After the powders and their binder have swished around for a certain length of time, the chalk forms a kind of dough which looks like many small balls about the size of marbles. These little dough balls are then machine-pressed into a large, long shape, just like a solid cylinder. This shape is called a cartridge. As a result of this pressing, air is forced out of the dough, and the moist particles in the dough are very closely bound to one another causing the chalk to be heavy and smooth textured.The cartridge, which is still damp and pliable, is then inserted into another machine called an extrusion press, where it is forced through a small tube. As the long rope of wet chalk comes out, an automatic slicer cuts it into many pieces. These pieces roll down to a tray, and are ready to be cut again into regular size sticks of chalk 80 mm. Since the pieces are still quite moist, they must be dried in large ovens, called kilns, before they become hard enough to be packed.Extruded chalk was described as "dustless". In the case of extruded chalk, however, the dust particles are weighted to fall straight down instead of flying through the air to make dust. In other words, the dust in "dustless" chalk is chemically controlled.Both molded and extruded chalks are manufactured in a variety of colors and shapes. There are round sticks, long square ones, Triangle, and their uses are varied. Chalks made for school and home are different from those needed in an art studio or factory; and just as in a family, no two members are alike even though they may resemble one another.
Chalk that is intended for the classroom must undergo stringent tests in order to perform well and be labeled nontoxic. All incoming materials are tested for purity before being used. After the chalk has been made into sticks, one stick from each batch is selected for tests. The density and break strength of the sample stick are determined. The sample is then used to write with, and the quality of the mark is studied. Erasability is also studied. First, the chalk mark is erased using a dry eraser, and the quality of erasure is examined. Then, the chalkboard is washed, and again the amount of chalk left on the board is examined. Furthermore, a sample from each batch is kept for five years so that it can be inspected if in the future its quality is questioned.
Many people consider using chalk and chalk-boards to present material outdated. Some experts claim that teachers have stubbornly resisted new technologies that could improve teaching—and eliminate the chalkboard entirely. A study which recently investigated whether teaching with overhead projectors was more effective than using chalkboards concluded that chalkboards were more interactive, progressive, and fruitful.A development much in the educational news lately is the electronic chalkboard. In place of a regular chalkboard, a teacher uses a large TV screen, inputting materials from a computer terminal. In a more advanced scenario, each student uses a terminal, to which the teacher sends information from a master computer. Experts claim that such set-ups are more visually exciting to students, more versatile than the old-fashioned chalkboards, cleaner than dusty chalk, easier for the teacher to use, and better able to present more complex material through the use of graphics and animation. Many studies on the feasibility of electronic chalkboards have been made, however, and most seem to favor keeping the traditional chalkboard, at least for now. Electronic chalkboards that are sophisticated and easily readable lie beyond the budget constraints and technological capabilities of most schools. Further, studies of the electronic system's effectiveness report that teachers who use it spent more time preparing their lessons, teachers and students were less interactive, students were dissatisfied with the electronic chalkboards, and the new devices divided the students' attention between the screen and the teacher conveying the information. Although the enthusiasm for electronic blackboards in some areas remains high, chalk use in the classroom is guaranteed for some time to come.